Hydroforming, a process in which single piece, generally cylindrical steel blanks are expanded within a die cavity under great internal pressure to produce non cylindrical frame rails and the like, is finding greater and greater production use. A recent development which has greatly increased the utility of the process is so called hydropiercing, in which holes and slots can be cut through the surfaces of the pressure formed part right in the die, so as to avoid the necessity of later hole cutting steps. An example of hydropiercing can be seen in co assigned U.S. Pat. No. 5,0398,533 issued Mar. 21, 1995 to Shimanovski et al., where a flat surface on a hydroformed part is pierced by allowing the highly pressurized internal fluid to blow out through a sharp edged die button, removing a slug of metal as it escapes to leave behind a hole shaped like the die button edge. It is necessary that the perimeter of the cutting edge of the die button be surrounded by an "O" ring, which is inset into a retention groove. The "O" ring seal is firmly pressed into the part surface, surrounding the area to be cut through. The "O" ring acts as a face seal to prevent the escape of pressurized fluid as the hole is cut. Production of the die button itself, including the machining of the "O" ring retention groove, is a simple process when the part surface surrounding the hole to be cut is flat. In that case, the die button surface and groove are also correspondingly flat. When the hole is to be cut through a non flat, trough like surface, manufacture of the die button is more difficult. While it is relatively simple to machine the basic surface of the die button to match the part surface, there is no known way to easily machine the "O" ring retention groove down into that complex, non flat surface, especially where the groove must pass through concave curved transition areas or "valleys". The machining process is complicated by the fact that the ideal groove cross section should have undercut shoulders on each side so as to retain the round cross sectioned "O" ring in the groove with a "snap" fit around the sides of the "O" ring.
One known U.S. Pat. 4,786,219 issued Nov. 22, 1988 to Oberlin et al., does disclose a method for machining a continuous groove into the outer surface of an elliptical tube. Such an exterior surface is everywhere convex, however, with no concave transition areas. A flat bottomed machining tool is disclosed, which is moved around the cutting path, and maintained at both a constant cutting depth relative to the surface and at substantially a perpendicular orientation relative to the surrounding surface. Those tool conditions would be both givens for any such machining process, of course. The primary focus of the patent is maintaining the tool at a constant cutting depth. However, the flat bottomed tool disclosed would simply not work if used in a curved surface like that disclosed in the subject invention, as it would interfere or bind drag when moved through the concave, sharply radiused transition portions of the cutting path.